The present invention relates to fluorescent lamp phosphors, and particularly relates to a method of preparing a manganese activated zinc orthosilicate phosphor and the phosphor produced thereby.
Manganese activated zinc silicate phosphors emitting in the green region of the visible spectrum are used, for example, in fluorescent lamps and cathode ray tubes. Prior art methods of preparing manganese activated zinc silicate phosphors ensure reactivity between the zinc and the manganese by various means. Such reactivity depends on efficient and complete incorporation of the dopant concentration levels of manganese ion used in the phosphor. For example, some processes have used an excess quantity of silica in the starting formulation to assist such reactivity. However, unreacted components in a phosphor can be detrimental to the light output of a lamp, although an excess amount of unreacted silica is less detrimental than excess metal oxide because of the metal oxide's greater absorption of ultraviolet radiation. Other processes use additives to facilitate a complete reaction. For example, magnesium difluoride may be used as a flux, or tungsten trioxide may be added to improve lamp maintenance, i.e., to lessen the loss of light output over the life of the lamp. The tungsten trioxide may be added to a silica containing formulation, or it may be added to a silicic acid based formulation with or without 0.01-1.0% of colloidal silica to reduce blending time. Other processes have added ammonium chloride and ammonium fluoride as reaction agents. Using additives to assist the reaction, however, can increase the particle size of the asfired phosphor.
Manganese activated zinc silicate phosphors must be reduced to a small particle size for use in either lamp applications or tagging applications. In particular, tagging applications require a fine particle size phosphor, i.e., less than 5 .mu.m, to avoid abrasion of the tagging equipment by the phosphor particles. The term "tagging" is used herein to refer to the application of a phosphor to an item for identification purposes. The phosphor may be applied directly to the item, e.g. to fabric or paper, or may be added to a coating or ink for application to the item.
Known manganese activated zinc silicate phosphors can have an average milled particle size of 5-7 .mu.m when the silicon source in the starting formulation is silicic acid, the most commonly used silicon starting material in phosphor production. It is difficult to achieve particle sizes small enough for use in tagging machines with the silicic acid derived phosphors. Extensive further milling of these prior art phosphors is required. Even for use in an ordinary fluorescent lamp, the silicic acid derived phosphor must be vigorously milled. However, such extensive milling alters the surface characteristics of the phosphor crystals, decreasing the brightness of the phosphor.
It would be desirable to have a green emitting manganese activated zinc silicate phosphor exhibiting a particle size of less than 5 .mu.m without significant loss of lamp brightness. It would also be desirable to provide improved control of the final particle size and morphology, and to achieve this control and small particle size at lower firing times and temperatures than are necessary with silicic acid derived phosphors. The phosphor and process described herein were developed to address that need.
Accordingly, it is an object of this invention to provide a process for producing a manganese activated zinc orthosilicate phosphor utilizing an ultrafine silica component as the silicon source in the starting formulation.
It is another object of this invention to provide a process for producing a manganese activated zinc orthosilicate phosphor providing improved control of the final particle size and morphology of the phosphor.
It is a further object of this invention to produce a manganese activated zinc orthosilicate phosphor having a primary particle size of less than 5 .mu.m and a plaque brightness equivalent to that of a commercially available manganese activated zinc orthosilicate phosphor.